1. Field of the Invention
The present invention relates to an optical recording medium and a tracking apparatus adapted for the same.
2. Description of Related Art
Accompanying the recent progress toward higher performance and personalization in the field of information apparatus such as computers, communication equipment and video equipment, the amount of information to be processed has been ever increasing and there has existed an extremely high demand for large increase in the density and capacity of a medium for recording information. In these circumstances, the system of reading the recorded information from the medium by an optical method will be considered to constitute the most powerful technique even in the future.
Presently, as the so-called optical recording media of the type adapted for the reading of recorded information by such optical method, a musical compact disc (CD), laser disc (LD), CD-ROM or magneto-optical disc capable of higher density recording of information has, for example, been put in practical use and is coming into wide use.
Such an optical recording medium or rotating disc has a track format in which tracks are formed concentrically or spirally in the circumferential direction and pits, each consisting of a recording unit of information, are recorded in the form of patterns along the tracks.
When reading the information recorded along the tracks, the light from a light source, e.g., a semiconductor laser is focused into a tiny spot of light through an optical head and projected onto the disc so that the variations in the amount of reflected light or transmitted light or the variations in the plane of polarization in the case of the magneto-optical disc are detected to generate time sequence signals corresponding to the presence and absence of the pits.
By reducing the spot size of the light beam focused on the tracks up to about the diffraction limit determined by its wavelength, it is permitted to reduce the size of the recording unit (pit) of information to substantially the spot size and thereby to realize a higher recording density.
On the other hand, when reading the information from such optical recording medium, during the rotation of the disc the light spot must be caused to accurately follow the fine pitch of the tracks, that is, the tracking control must be effected. In order to optically discriminate for this purpose the tracks adjoining in the radial direction of the disc from each other, the concentric or spiral tracking guides, which are generally called guide grooves, are continuously formed along the tracks to provide the reflected light or transmitted light of the light spot with an inherent optical characteristic. In this case, in a typical example, an examination of the disc cross-sectional construction in the direction crossing the tracks shows that the stepped structures formed by the guide grooves and the intervening land portions determine the track pitch P and hence the track width.
On the other hand, another construction in which the tracking guides are discontinuously formed along the tracks is also made known by for example the composite track servo system adapted to compensate an offset caused in a tracking error signal due to the inclination of the disc, and in this case the intermittent patterns of the tracking guides are formed in such a manner that the tracks adjoining with respect to the radial direction of the disc coincide in the phase of the intermittent patterns with each other.
The tracking control is accomplished by the tracking servo technique so that the focused tiny light spot is projected on the guide groove of the optical recording medium from the semiconductor laser through the optical head and the variation in the light quantity of the resulting reflected light or transmitted light or the diffraction pattern is photoelectrically detected, thereby servo-controlling the position of the objective lens of the optical head so as to prevent deviation of the light spot from the track. Where the tracking guides are in the intermittent patterns, the sampling method is utilized for the detection of tracking error signals.
The tracking servo techniques are roughly divided into two known methods. One is a method employing a single light spot and in this case the tracking error detecting method used is the well known push-pull method, the heterodyne method or the like
The other method is a so-called three beam method employing three light spots arranged in a direction which is slightly inclined relative to the track lengthwise direction. In this case, the light beam from the semiconductor laser is separated by a diffraction grating into a main beam or the zero-order beam and two sub beams or the .+-. first-order beams. The main beam produces a recording/playback main beam on the optical recording medium and two tracking sub spots are produced by the sub beams so as to interpose the main spot therebetween.
Also, recently there has been proposed a super resolution optical recording/playback system which utilizes the optical or thermal nonlinear characteristics of an optical recording medium, that is, a system capable of reading the information recorded with a pit size which is less than the diffraction limit of a light beam.
Then, the conventional optical recording media are disadvantageous in that due to the necessity for the previously mentioned tracking control utilizing the diffraction phenomenon of the light beam, it is impossible to make the track width or the track pitch less than the diffraction limit of the light beam used for recording and/or playback purposes.
In other words, if the track width is decreased, the track pitch determined by the arrangement spacing of the guide grooves is also inevitably decreased, whereas if the track pitch P is made less than the diffraction limit P.sub.c of the light beam (P.sub.c =.lambda./2NA:.lambda. is the wavelength of the light and NA is the numerical aperture of the objective lens of the optical head), irrespective of the position in which the light spot is placed relative to the guide groove, the light quantity of the reflected light returning to the objective lens of the optical head is no longer varied and it is no longer possible to generate a tracking error signal from the optical head.
In this case, in order to positively detect photoelectrically the backward light quantity corresponding to the position of the light spot relative to the guide groove, the difference between the track pitch P and the diffraction limit P.sub.c of the light spot must be sufficient, and therefore it has been necessary in the past to select the track pitch P to be sufficiently large as compared with the diffraction limit P.sub.c for the purpose of obtaining fully reliable tracking error signals thus restricting the lower limit of the track pitch and thereby preventing increase in the density of optical recording media. Moreover, these deficiencies are similarly encountered in the case of the super resolution optical recording/playback system which utilizes the optical or thermal nonlinear characteristics of optical recording media.